1. Field of the Invention
The present invention relates to an oscillator and, more specifically to a crystal oscillator having a regulated, symmetrical amplitude to minimize non-linearities and deleterious stimulation of harmonic modes of oscillation.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art or conventional by virtue of their inclusion within this section.
Within nearly every electronic subsystem is some sort of generator that produces cyclical waveforms. The waveform generator is oftentimes referred to as an oscillator for producing regular oscillation signals. Depending on the application, an oscillator can be used simply as a source of regularly spaced pulses or clock signals. Oscillators are oftentimes rated depending on their stability and accuracy, frequency adjustability (i.e., tunability), and power consumption.
There are numerous types of oscillators in the marketplace. A simple kind of oscillator is an RC relaxation oscillator. More complex and stable oscillators involve the more popular LC oscillator. While LC oscillators are more stable than RC oscillators, a crystal oscillator is generally more stable than LC oscillators.
Crystal oscillators use a piece of quartz (i.e., glass or silicon dioxide) that is cut and polished to vibrate at a specified frequency. Quartz is piezoelectric, wherein acoustic waves in the crystal are driven by an applied electric field and, in turn, can generate a voltage at the surface of the crystal. The quartz thereby operates as a resonator that is pre-tuned to a specific resonant frequency. The resulting effect is that of a modeled RLC circuit that produces a rapidly changing reactance with frequency, with the RLC-modeled crystal providing positive feedback and gain at the resonant frequency, leading to sustained oscillations.
In order to initiate and maintain strain on the crystal, a crystal oscillator generally includes an amplifier coupled across nodes of the crystal. While the least impedance value across the crystal occurs at its resonant frequency, an amplifier that drives the crystal may “pull” the frequency of the crystal depending on certain performance traits of that amplifier. For example, an amplifier can be formed using bipolar transistors, field-effect transistors (FETs), or a combination thereof. Depending on how the transistors are manufactured and connected, the transistors can operate in both linear and non-linear voltage regions. The motional impedance of each possible mode of oscillation within the crystal and the current through the motional impedance ZM(i) is proportional to the amplitude of oscillation at mode i. Thus, as the amplitude of oscillation changes, the amplifier can go from a linear operation to a non-linear operation. For example, when a critical transconductance of a transistor within the amplifier is exceeded by applying a bias current thereto above a critical value, oscillation will increase.
Non-linear effects start appearing when the amplitude of the sinusoidal driving voltage of the transistor becomes so large as to generate harmonics in the output current from the amplifier. The non-linear effects and resulting induced harmonics within the crystal predominate if the transistors operate outside a weak inversion range and, thus, within what is known as the “saturation range.” If operated in the non-linear regions of strong inversion or saturation, the overall stability of the crystal will be negatively impacted. Harmonics are created across the crystal to distort the driving voltages across ZM. These harmonics are then intra-modulated in the device non-linearity, eventually creating an additional fundamental component with a different phase that shifts the frequency of oscillation. If, for example, a harmonic is produced that is close enough in frequency to an unwanted mode of vibration i of the crystal, the oscillation frequency will be at the unwanted mode i, provided there is sufficient gain to sustain oscillation. Non-linear effects can also appear if the output current of the amplifier is not symmetrical for a given input voltage amplitude. The current the amplifier can source for negative input voltage amplitudes should desirably be equal in magnitude to the amount of current the amplifier can sink for positive input voltage amplitudes of the same magnitude. Harmonics can be reduced with increased amplifier symmetry.
It would be desirable to utilize the benefits of a crystal oscillator, but without the non-linear effects that cause deleterious harmonic components to exist within the crystal. It would, therefore, be desirable to introduce an amplifier that operates with the positive feedback from the crystal that is regulated to operate with symmetrical voltages and currents across the crystal nodes to promote stability and diminish stimulation of harmonic modes of oscillation.